Cross-linked poly(dimethylaminoethyl acrylamide) coated magnetic nanoparticles: a high loaded, retrievable, and stable basic catalyst for the synthesis of benzopyranes in water

Abstract

A novel heterogeneous catalyst has been synthesized based on the distillation–precipitation–polymerization of methyl acrylate onto modified magnetic nanoparticles followed by the amidation of the methyl ester groups using N,N-dimethylethylenediamine. The resulting poly(dimethylaminoethyl acrylamide) coated magnetic nanoparticles (MNP@PDMA) catalyst was characterized using an array of sophisticated analytical techniques, including FT-IR, TGA, SEM, TEM, CHN, vibrating sample magnetometer (VSM), and XRD analysis. The resulting heterogeneous base catalyst allowed the performance of a domino Knoevenagel condensation/Michael addition/cycloaddition reaction toward the synthesis of 4H-benzo[b]pyranes in excellent yields using water as the reaction medium. The multilayered and polymeric identity of the coated material on the surface of the magnetic nanoparticles provides many catalytic units resulting in the high loading level and high stability of the catalyst. Straightforward magnetic separation and recycling of the catalyst for up to 5 runs is possible without any significant loss of efficiency.

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1. Introduction

The application of homogeneous catalysts in organic synthesis have advantages such as the accessibility of catalytic sites, tuning the chemo-, regio-, and enantioselectivity, good yields and easy optimization of the catalytic system by the modification of ligands and metals. However, they suffer from significant drawbacks such as instability, deactivation, and difficult recovery and recycling of the catalyst.^1,2 Therefore, the heterogenization of homogeneous catalysts is an applicable way to overcome these problems. Commonly, heterogenization is carried out by supporting the homogeneous catalyst onto the surface of an insoluble organic or inorganic solid such as cross-linked polymers,^3,4 mesoporous silica,^5,6 magnetic nanoparticles^7–9 and zeolites.¹⁰

Recently, magnetic supported catalysts have attracted considerable attention in organic synthesis as heterogeneous catalysis.^7–9,11 Surface modification followed by supporting the catalyst onto magnetic nanoparticles is an elegant way to bridge the gap between heterogeneous and homogeneous catalysis. The most highlighted aspect of magnetic nanoparticles is their easy separation from the reaction mixture by the application of an external permanent magnet. This made magnetic nanoparticles a promising alternative to other heterogeneous catalysts, especially porous/mesoporous systems most of which require a filtration or centrifugation step or a tedious workup of the reaction mixture to recover the catalyst.^12,13 Magnetic nanoparticles also have good stability, easy synthesis and functionalization and a high surface area, as well as low toxicity and cost.^14–16 To date, many homogenous catalysts have been immobilized onto magnetic nanoparticles via different surface functionalization processes.¹¹

Although these heterogeneous catalysts are widely used in organic synthesis, they are in most cases not as active and selective as their homogeneous counterparts.¹⁷ A significant proportion of these catalysts are deep inside the supporting material and thus reactants have limited access to their catalytic sites.¹⁸ In addition, it is obvious that the number of catalytic units is low when compared to the large surface of the magnetic nanoparticles (low loading), which leads to the use of a larger amount of catalyst.^19–21 Surface functionalization of the magnetic nanoparticles with various polymer grafting methods is a good choice to overcome these limits.^22–24 The surface-grafted polymers not only efficiently improve the solubility of the magnetic nanoparticles in different solvents but also endows the particles with a series of functionalities for various applications. This also provides the advantage that more catalytic units can be grafted to the surface of magnetic nanoparticles and thus provides a higher catalyst loading. Accordingly, different types of polymers with various topological structures, including linear,²⁵ brush,²⁶ hyper-branched²⁷ and dendrimer,^27–29 have been used for the polymeric functionalization of magnetic nanoparticles.

In our present work, we report the development of a novel heterogeneous organocatalyst combining the unique properties of magnetic nanoparticles and a cross-linked basic polymer. The broad applicability of the ensuing catalyst was investigated through a library synthesis of 2-amino-4H-benzo[b]pyranes.

4H-Benzo[b]pyranes have a wide range of biological activities and are widely distributed in nature. They also have diverse pharmacological properties and some significant applications in the design of new therapeutic agents.^30,31 The conventional method reported for the synthesis of 4H-benzo[b]pyranes uses organic solvents, such as DMF or acetic acid, under reflux.³² Several homogeneous or heterogeneous basic or acidic catalysts have been used for this reaction.^{31,33,36–39}

However, in most of these methods, the catalysts cannot be recycled and a large volume of solvent is required for product separation. In addition, they have one or more of the other disadvantages such as using an equivalent or high wt% of catalyst, toxicity, the use of centrifugation or filtration methods for catalyst recovery, long reaction times, the use of reflux conditions under an inert atmosphere, non-green solvents, and a high overall cost of reagents. Thus, new routes for the synthesis of 4H-benzo[b]pyranes have attracted considerable attention in the search for new and rapid methods to access them.

2. Experimental

2.1. Reagents and analysis

Ferric chloride hexahydrate (FeCl₃·6H₂O), ammonia (25%), ferrous chloride tetrahydrate (FeCl₂·4H₂O), 3-(trimethoxysilyl)propylmethacrylate (MPS), N,N-dimethylethylenediamine and methylenebisacrylamide (MBA) were obtained from Merck and were used without any further purification. 2,2′-Azobisisobutyronitrile (AIBN, Kanto, 97%) was recrystallized from ethanol, and methyl acrylate (Sigma-Aldrich) was distilled prior to use.

Thin layer chromatography (TLC) was performed on silica gel 60 F254 plates and UV light was used for visualization. FT-IR spectra of samples were recorded using an ABB Bomem MB-100 FT-IR spectrophotometer. NMR spectra were recorded on a Bruker NMR 500 MHz instrument. Thermogravimetric analysis (TGA) was acquired under a nitrogen atmosphere with a TGA Q 50 thermo-gravimetric analyzer. Transmission electron microscopy (TEM) images were taken with a TOPCON-002B electron microscope. The magnetic properties of the catalyst were measured by a VSM Model 7400.

2.2. Synthesis of the modified magnetic nanoparticles (MNP@MPS)

The magnetic Fe₃O₄ nanoparticles were prepared using a chemical co-precipitation method. An iron salt solution was obtained by mixing 13.6 g of FeCl₃·6H₂O and 5 g of FeCl₂·4H₂O in 500 mL of deionized water under nitrogen at room temperature. The pH was adjusted to 12 by the dropwise addition of NH₃ solution. A black precipitate was formed after continuously stirring for 1 h, which was magnetically separated and washed four times with deionized water and two times with ethanol, and then dried under vacuum at 50 °C overnight.

The resulting Fe₃O₄ nanoparticles (3 g) were ultrasonically suspended in 400 mL of an ethanol–water mixture (4 : 1) and the pH of the solution was adjusted to 9 by adding NH₃ solution. Tetraethyl orthosilicate (TEOS, 15 mL) was added dropwise to the solution at 50 °C in the presence of a constant nitrogen flow. The mixture was stirred for 6 h. Then, the silica coated nanoparticles were magnetically separated and washed three times with deionized water and two times with ethanol. The final dark brown product (MNP) was dried at 50 °C under vacuum for 24 h.

Afterward, 1 g of MNP were dispersed in 40 mL of a 4 : 1 mixture of ethanol–water, and then 2 mL of ammonium hydroxide 25% solution was added. Then, an excess amount (10 mmol per 1 g of MNP) of the MPS was added dropwise over a period of 10 min, and the reaction mixture was stirred at 50 °C for 48 h. The modified MNP@MPS were magnetically separated and washed three times with methanol to remove any excess of reagents and salts.

2.3. Synthesis of the catalyst

500 mg of MNP@MPS nanoparticles were dispersed by ultrasonication in 200 mL of methanol in a 500 mL single-necked flask for 10 min. Then, a mixture of 2 g of methyl acrylate, 500 mg of MBA, and 70 mg of AIBN were added to the flask to initiate polymerization. The mixture was completely deoxygenated by bubbling purified argon for 30 min. The flask was submerged in a heating oil bath attached with a fractionating column, Liebig condenser, and a receiver. The reaction mixture was heated from ambient temperature to reflux within 1 h and the reaction was complete within 5 h when about 100 mL of methanol was distilled from the reaction mixture. The cross-linked poly(methyacrylate) coated MNP (MNP@PMA) were collected by magnetic separation and washed two times with water and three times with methanol to remove excess reactants and the few generated co-polymer microspheres.

500 mg of MNP@PMA nanoparticles were dispersed by ultrasonication in 20 mL of methanol in a 50 mL single-necked flask for 10 min. Then, 15 mL of N,N-dimethylethylenediamine was added and the mixture was stirred at 60 °C until the stretching vibration bond of the carbonyl group in poly(methyacrylate) disappeared (about 48 h). The final catalyst poly(dimethylaminoethyl) acrylamide coated MNP (MNP@PDMA) was magnetically separated and washed three times with methanol to remove any excess of reagents and then dried at 60 °C for 12 h.

2.4. General procedure for the synthesis of 4H-benzo[b]pyranes

In a 10 mL round bottomed flask, aldehyde (1 mmol), malononitrile (1.2 mmol), water (2 mL), and MNP@PDMA (25 mg, 4 mol%) were added and the mixture was stirred for 5 min at room temperature. Then, dimedone (1 mmol) was added to the flask and the reaction mixture was vigorously stirred for an appropriate time monitored by TLC until all the starting materials disappeared. The reaction mixture was diluted with hot ethyl acetate. Then, the catalyst was magnetically separated and washed two times with methanol and dried under vacuum. The organic layer was concentrated under vacuum and the resulting solid products were recrystallized from ethanol.

3. Results and discussion

A schematic illustration of the catalyst synthesis is shown in Scheme 1. The first step involves the synthesis of dark brown MNPs using a co-precipitation technique based on a reported method.³⁴ The Fe₃O₄ surface was coated with silica using TEOS, which increased its stability against heat and harsh reagents. A coating of 3-methacryloxypropyltrimethoxysilane (MPS) onto the MNPs gives two important results including easier polymerization on the surface of magnetic nanoparticles because of the presence of the methacrylate groups and the covalent attachment of copolymers onto the surface of MNP. Without MPS modification, only a minor part of magnetic nanoparticles can be coated with a complete layer of polymer chains instead of all of them.³⁵ MNP@PMA was synthesized via a distillation–precipitation–polymerization method without any surfactant with MNP@MPS as the seed, methyl acrylate as the monomer, MBA as the cross-linker, and AIBN as an initiator in methanol. Because cross-linked polymers are not soluble in methanol, they will continuously precipitate from the solution and attach to the surface of the MNPs to form a robust shell through the simultaneous distillation of the solvent. This surfactant-less method leads to the formation of a multilayered polymeric shell around the MNPs. In the final step, the reaction of the ester groups with N,N-dimethylethylenediamine provides the desired MNP@PDMA catalyst. The tertiary amine groups on the surface of the catalyst can act as a basic catalyst (Scheme 1).

Scheme 1 Synthesis of the MNP@PDMA catalyst.

The identity of the catalyst was characterized using an array of sophisticated analytical techniques. Fig. 1 shows the FT-IR spectra of Fe₃O₄@SiO₂ (MNP) (a), MNP@MPS (b), MNP@PMA (c), and MNP@PDMA (d). Fig. 1a shows the stretching vibrations of Fe–O and Si–O at 582 and 1083 cm⁻¹, respectively. The MPS coating on the MNP was confirmed by the peaks observed at 2929, 1707, and 1404 cm⁻¹ in Fig. 1b, which were attributed to C–H, ester C=O, and C=C bonds, respectively. The FT-IR spectrum of MNP@PMA (Fig. 1c) shows a strong vibration bond in 1735 cm⁻¹ corresponding to the ester C=O of poly(methyacrylate). Conversion of the CO₂Me groups in the polymer shell to CONH(CH₂)₂N(Me)₂ and the formation of the final catalyst were confirmed by the appearance of an amide C=O bond at 1638 cm⁻¹ and elimination of the ester C=O peak in the FT-IR spectrum of MNP@PDMA, as shown in Fig. 1d (and compared to Fig. 1c).

Fig. 1 FT-IR spectra of MNP (a), MNP@MPS (b), MNP@PMA (c) and MNP@PDMA (d).

The presence as well as the degree of crystallinity of magnetic iron oxide (Fe₃O₄) in the synthesized MNP@PDMA was obtained using X-ray diffraction (XRD) patterns (Fig. 2). According to the XRD pattern of a standard Fe₃O₄ sample (JCPDS file no. 19-0629), the pattern of MNP@PDMA is completely matched with pure Fe₃O₄, confirming the presence of Fe₃O₄ and denotes that all modifications did not change the Fe₃O₄ phase. The broad peak at 2θ = 20 corresponds to amorphous silica phase in the catalyst structure.

Fig. 2 XRD diffraction patterns of the MNP@PDMA catalyst.

The magnetization curves of bare Fe₃O₄ nanoparticles and the MNP@PMDA catalyst are shown in Fig. 3a and b, respectively. It can be seen that the Fe₃O₄ and MNP@PDMA particles are superparamagnetic with magnetic saturation (MS) values of about 64.1 and 24.6 emu g⁻¹, respectively. The 39.5 emu g⁻¹ diminution of saturation magnetization was attributed to the presence of the SiO₂/PDMA shell on the Fe₃O₄ particles. However, the level of catalyst magnetization is still sufficient to strongly respond to an external magnet resulting in complete catalyst recovery (Fig. 3, inserted image).

Fig. 3 VSM curves for Fe₃O₄ (a) and MNP@PDMA (b) and magnetic recovery of the catalyst (inserted image).

Thermogravimetric analysis (TGA) of the magnetic nanocatalyst MNP@PDMA as well as MNP@MPS and MNP@PMA were performed over the temperature range of 25–900 °C (Fig. 4a–c). As shown in Fig. 4a–c, weight loss within 150 °C was attributed to the loss of adsorbed water molecules on the surface of the nanostructured materials. The TGA curve of MNP@MPS (Fig. 4a) shows a weight loss at 250 °C, which was attributed to the loss of propylmethacrylate. From this weight loss (4.1 wt%), the loading of MPS bound to the MNP surface was calculated to be 0.32 mmol g⁻¹. The TGA curve of MNP@PMA (Fig. 4b) shows an increased weight loss (21.5 wt%) due to the loss of the PMA shell. MNP@PDMA loses 29% of its weight at 220 °C (Fig. 4c), corresponding to the cross-linked poly(dimethylaminoethyl) acrylamide. Because both MNP@PMA and MNP@PDMA contain a monomer and cross-linker, the calculation of an accurate loading amount of –CO₂Me or –NMe₂ based on the TGA curves was not possible.

Fig. 4 TGA curves for (a) MNP@MPS, (b) MNP@PMA, and (c) MNP@PDMA.

The loading amount of basic NMe₂ groups on the surface of the catalyst was calculated using a back titration method. 50 mg MNP@PDMA was dispersed in water and an excess amount of HCl (0.5 M) was added. The mixture was stirred for 3 h until all the amine groups on the surface of the catalyst neutralized. Then, the catalyst was magnetically separated. The excess amount of HCl remaining in the solution was titrated against NaOH (0.1 M) in the presence of phenolphthalein. The loading amount of NMe₂ on the surface of MNP@PDMA was found to be 1.58 mmol g⁻¹.

The results of the elemental analysis for MNP@MPS, MNP@PMA and MNP@PDMA are shown in Table 1. The loading amount of MPS on the surface of MNP@MPS was calculated to be about 0.38 mmol g⁻¹ based on the C atom content, which is in good agreement with the TGA data. Because the only source of N atoms in MNP@PMA is the nitrogen of the cross-linker (plus 0.09% in Entry 1), the content of MBA in MNP@PMA can be calculated based on the N atom content. It was found that the loading amount of MBA in MNP@PMA was about 0.28 mmol g⁻¹, i.e. 4.3 wt% of MNP@PMA. The loading amount of CO₂Me groups was also calculated based on the C content of MNP@PMA after subtracting the C content of MBA (%C_{for MBA} = 2.35 wt%). As shown in Table 1 (Entry 3), the nitrogen content of MNP@PDMA increased due to the conversion of PMA to PDMA on the catalysts surface. Because the cross-linker content of MNP@PMA and MNP@PDMA is almost the same, the loading amount of N,N-dimethylaminoethyl acrylamide in PDMA and subsequently basic NMe₂ group in MNP@PDMA can be calculated. Regarding the N content of MNP@PMA (0.87 − 0.09 = 0.78%), corresponding to MBA, it can be calculated that the N content of N,N-dimethylaminoethyl acrylamide in PDMA is 4.18% (5.05 − 0.78 − 0.09 = 4.18%), which is equal to 1.49 mmol g⁻¹ of N,N-dimethylaminoethyl acrylamide or NMe₂. These results are in good agreement with the loading amount calculated from our titration studies.

Table 1 Results of CHN analysis for MNP@MPS, MNP@PMA and MNP@PDMA

Entry	Sample	C (%)	H (%)	N (%)	Loading (mmol g^−1)
a Based on carbon atom content.b Based on nitrogen atom content.c Based on carbon atom content after subtracting the %C of MBA. %C of MBA can be calculated from the loading amount of MBA (0.28 mmol g^−1), which is 2.35%.d Based on nitrogen atom content after subtracting the %N of MBA, which is 0.78% (0.87 − 0.09 = 0.78%).
1	MNP@MPS	3.19	0.62	0.09	0.38^a
2	MNP@PMA	13.27	1.97	0.87	For MBA 0.28^b
					For CO2Me 1.61^c
3	MNP@PDMA	18.15	3.09	5.05	For NMe2 1.49^d
4	Recycled MNP@PDMA	17.63	2.94	4.86	For NMe2 1.43^d

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The morphology of the MNP@PDMA catalyst was also investigated by TEM and SEM (Fig. 5a and b, respectively). The TEM image of the MNP@PDMA catalyst confirms the nanometer size of the catalyst and shows that dark MNPs with 5–10 nm diameters are encapsulated by grey polymer layers. The SEM and TEM images of MNP@PDMA show that the MNPs are entrapped by polymer shells.

Fig. 5 TEM (a) and SEM (b) images of the MNP@PDMA catalyst.

To evaluate the catalytic performance and efficiency of MNP@PDMA, we selected the one-pot reaction of benzaldehyde, malononitrile and dimedone for the synthesis of 2-amino-4H-benzo[b]pyrane as a base-catalyzed model reaction.

The reaction of benzaldehyde, malononitrile and dimedone without catalyst under solvent-free or using water as a solvent at 80 °C gives only a trace amount of 4H-benzo[b]pyrane in 60 min (Table 2, Entries 1 and 2). Thus, our first investigation focused on the optimization of solvent and temperature in the presence of enough catalyst (Table 2). It was found that the simultaneous presence of solvent and catalyst has a key role. The reaction gives trace amounts of product without solvent even with 10 mol% of catalyst. Optimization of solvent and temperature in the presence of enough catalyst showed that H₂O was the best medium for the reaction at room temperature. Then, the amount of catalyst was optimized using H₂O as the solvent at room temperature in the model reaction. As indicated in Table 2, the maximum yield of 4H-benzo[b]pyrane was obtained using 25 mg of catalyst (4 mol%) after 25 min at room temperature (Entry 14). A lower amount of catalyst dramatically decreased the reaction yield despite increasing the reaction time or temperature (Entries 13 and 16).

Table 2 Optimization of solvent, temperature and catalyst amount for the synthesis of amino 4H-benzo[b]pyrane^a

Entry	MNP@PDMA (mol%)	Solvent	T (°C)	Time (min)	Yield^b (%)
a Reaction condition: benzaldehyde (1 mmol), malononitrile (1.2 mmol), dimedone (1 mmol), solvent (3 mL), loading of amine groups in MNP@PDMA (1.58 mmol g^−1).b Isolated yield.
1	—	Solvent-free	80	60	Trace
2	—	H2O	80	60	15
3	10	Solvent-free	r.t.	60	20
4	10	EtOH	r.t.	60	60
5	10	EtOH : H2O (1 : 1)	r.t.	60	84
6	10	H2O	r.t.	60	94
7	10	H2O	50	40	94
8	10	CH3CN	50	60	80
9	10	CH2Cl2	r.t.	60	21
10	10	H2O	r.t.	10	85
11	6	H2O	r.t.	10	80
12	4	H2O	r.t.	10	77
13	3	H2O	r.t.	10	59
14	4	H2O	r.t.	25	95
15	3	H2O	r.t.	30	75
16	3	H2O	60	30	84

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Thus, the reaction of benzaldehyde, malononitrile, and dimedone proceeds well at room temperature in the presence of 4 mol% of catalyst and H₂O as the solvent via an aldol condensation/Michael addition/intermolecular cycloaddition cascade reaction and produces the corresponding 4H-benzo[b]pyrane within 25 minutes in 95% yield.

With the optimized reaction conditions, we examined the generality of the supported MNP@PDMA catalyst toward the synthesis of 4H-benzo[b]pyranes using different aldehyde substrates. As seen in Table 3, various aldehydes such as aldehydes carrying electron-withdrawing groups (Entries 1–9), weak and strong electron-donating groups (Entries 9–14, 18), a conjugated aldehyde (Entry 15) and 1,4-dialdehyde (Entry 16) also reacted and gave good yields of the desired product. However, in the presence of 4 mol% of MNP@PDMA, the reaction time for highly deactivated aldehydes increased. In a scale-up experiment, we examined the reaction between benzaldehyde, malononitrile and dimedone as a model reaction under optimized conditions on a 10 mmol scale. The yield of the scale-up reaction did not change. This demonstrated that the catalyst performance in the scale-up process was comparable to that found in the 1 mmol scale reaction.

Table 3 Synthesis of 2-amino-4H-benzo[b]pyranes under the optimized conditions^a

Entry	Ar	Time (min)	Yield^b (%)
a Reaction condition: aldehyde (1 mmol), malononitrile (1.2 mmol), dimedone (1 mmol), H2O (3 mL), MNP@PDMA (25 mg, 4 mol%), room temperature.b Isolated yield.
1	2-NO2-Ph	12	91
2	3-NO2-Ph	10	95
3	4-NO2-Ph	10	96
4	4-Cl-Ph	13	92
5	2-Cl-Ph	10	90
6	2,4-Di(Cl)Ph	15	92
7	4-(CN)-Ph	21	89
8	Ph	25	95
9	4-Me-Ph	25	89
10	2-(OH)-Ph	31	80
11	4-(OH)-Ph	33	86
12	2-(OMe)-Ph	35	75
13	4-(OMe)-Ph	45	86
14	4-(NMe2)-Ph	40	81
15	PhCH=CH–	41	88
16	Furfural	30	90
18	Vanillin	45	84
19	4-(CHO)-Ph	32	87

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Although we have not established the mechanism of the catalytic reaction in an experimental manner, a possible explanation is proposed in Scheme 1. It is notable that the heterogeneous basic catalyst is partially protonated in H₂O (Scheme 2).

Scheme 2 Plausible mechanism for the synthesis of the 4H-benzo[b]pyrane products through the basic site of the MNP@PDMA catalyst.

The recyclability of MNP@PDMA was investigated in the synthesis of 4H-benzo[b]pyranes by selecting the reaction between benzaldehyde, malononitrile and dimedone as a model reaction. The reaction was repeated five times and after each cycle, the catalyst was magnetically separated, washed and dried for the next run. The results indicate that no significant loss of catalytic activity was observed and the yield of the product remained up to 90% (Fig. 6). It is notable that the amount of catalyst remaining after 5 runs was measured to be 21 mg. In addition, CHN analysis showed that the catalyst content after five cycles did not significantly change (Table 1, Entry 4). These results show that the catalyst is stable under the reaction conditions and no significant leaching occurs.

Fig. 6 Recycling experiment.

To determine whether our reaction using MNP@PDMA actually occurs through a heterogeneous catalytic process, the following experiment was carried out. The model reaction was performed under the optimized conditions (Table 2, Entry 14), and the MNP@PDMA catalyst was completely removed from the reaction mixture using an external magnet after 10 min. Then, the reaction mixture was allowed to stir for 60 min (Fig. 7) and it was found that no further production of 4H-benzo[b]pyrane was observed. Moreover, the titration analysis of the recycled catalyst showed that the loading amount of NMe₂ did not significantly change (1.51 mmol g⁻¹). These results can rule out any contribution of leached amine into the reaction medium and confirms that the observed catalyst is completely heterogeneous.

Fig. 7 Leaching experiment. (a) Reaction in the presence of catalyst and (b) reaction after the removal of catalyst.

Table 4 summarizes the advantages of the MNP@PDMA catalyst compared to the most recently reported heterogeneous catalyst used for the synthesis of 2-amino-4H-benzo[b]pyranes. As seen, the present protocol and catalyst have several advantages such as magnetic recyclability, a high loading level of active catalyst sites, requiring a low weight percentage of catalyst and high product yield.

Table 4 Comparison of reaction conditions and catalyst efficiency of MNP@PDMA with the most recently reported catalyst used for the synthesis of 2-amino-4H-benzo[b]pyranes

Catalyst	Magnetic recoverable	Catalyst loading^a (mmol g^−1) per mg per mol%	Solvent	T (°C)	Time^b (min)	Yield^b (%)	Ref.
a For a 1 mmol scale reaction.b For benzaldehyde.
MNP@PDMA	Yes	1.58/25/4	H2O	r.t.	25	95	This work
Amino-β-cyclodextrin	No	—/125/9	None	r.t.	1	95	31
Fe3O4@SiO2/DABCO	Yes	—/50/—	H2O	80	25	90	39
SB-DABCO	No	1/60/6	EtOH	r.t.	35	96	36
Salep-g-PMAPTAH	No	3/2/0.5	H2O	r.t.	15	93	33
(SB-DBU)Cl	No	0.45/110/5	EtOH	r.t.	35	94	37
Nano γ-Fe2O3	Yes	—/16/10	H2O	r.t.	300	95	38

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4. Conclusions

We have developed a novel and efficient heterogeneous basic catalyst in which cross-linked poly(dimethylaminoethyl) acrylamide coated on modified MNPs were prepared using a distillation–precipitation–polymerization method. The resulting catalyst was proven to be an effective magnetic catalyst used for the green synthesis of 2-amino-4H-benzo[b]pyranes in excellent yield. The high loading level, high stability, high efficiency at room temperature in water, easy magnetic recyclability and reusability, short reaction time, excellent yields and ease of product separation are the most important aspects of the present catalyst and protocol. Moreover, we have developed a novel method for the synthesis of magnetic catalysts with high loading level and good stability, which have significant potential in both laboratory and industrial applications.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra07503j

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